Active noise cancellation is a method for reducing unwanted sound. Sound consists of vibrations in the air, which can be represented as a wave. If a speaker emits a sound whose wave has the same amplitude and an exact opposite polarity to the original sound, the waves cancel out and the result is no sound. A computer analyzes the waveform of the background aural or nonaural noise, then generates a similar waveform rotated 180 degrees out of phase to cancel background noise out by interference. This method differs from passive “noise cancellation” (sound proofing) such as insulation, sound-absorbing ceiling tiles, automobile mufflers or using headphones to suppress the noise. The advantages of active noise control methods compared to passive ones are: they are more effective, less bulky, and can be made to be selective, that is, to block unwanted noise (e.g. from an engine) but not useful sound (e.g. voice).
Active noise cancellation involves superimposing on a noise acoustic wave an opposite acoustic wave that destructively interferes with and cancels the noise acoustic wave. In active noise cancellation systems, the characteristics of the noise acoustic wave are sensed, a canceling acoustic wave is generated and delivered to a location through a speaker. The combined waves are monitored at the location and a feedback or error signal is produced for interactive adjustment of the cancellation of the noise acoustic wave.
Implementation of the active noise cancellation principle is arranged to accommodate changes in the frequency and intensity characteristics of the noise acoustic wave by incorporating adaptability into the feedback or error path of the active noise cancellation system.
Active noise cancellation systems generally provide only a relatively small and geographically fixed volume of “quiet space”. In a larger environment, this space would not accommodate the normal movement of an item or person within that larger space. Thus, with current technology, the quiet space is relatively small and fixed, and if the target moves outside this space, there is little or no reduction of noise.
U.S. Patent Application Publication No. US 2005/0226434, describes a noise reduction system employed in a working environment to reduce the noise that a user within that working environment experiences. The system determines the user's location within the working environment and produce a remedial noise profile that is configured to reduce the noise that the user experiences at the user's current location. The noise reduction system includes multiple speakers installed in the working environment. A signal that represents the remedial noise profile is used to drive one or more speakers proximate the user's current location so that the user experiences less noise at that location.
One disadvantage of the noise reduction system disclosed in the 2005/0226434 publication is that the system only can reduce noise by turning on the speakers proximate to the user's location when the user is relatively stationary at the location. The system cannot provide an effective noise cancellation for the user when the user is moving.
The apparatus and method for reducing noise for a moveable target of the present disclosure solves one or more of the problems set forth above.